Solid electrolytic capacitors (e.g., tantalum capacitors) have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. The anode of a typical solid electrolytic capacitor includes a porous anode body, with a lead wire extending beyond the anode body and connected to an anode termination of the capacitor. The anode can be formed by first pressing a tantalum powder into a pellet that is then sintered to create fused connections between individual powder particles. One problem with many conventional solid electrolytic capacitors is that there is resistance to current flow from the anode termination to the lead wire attached to or embedded in the anode body, which contributes to the equivalent series resistance (ESR) of the finished device. Resistances inside the body of the anode generate parallel resistances that also contribute to the ESR of the finished device. The current travels from the point of lead wire egress to the anode body to all points of the anode body through the path(s) of least resistance. The current must pass from the lead wire into the anode body through points of contact between the lead wire and the particles of the anode body. The current must then travel through the porous anode body, through small necks of the sintered particles.
Increasing the lead wire diameter decreases resistance in the wire itself and between the wire and the anode body. Unfortunately, increasing the wire diameter also reduces the capacitance by displacing porous anode body material that would otherwise contribute to capacitance. In attempt to solve this problem, attempts have been made to employ multiple lead wires based on the theory that the path length from points in the porous body become closer to at least one anode lead wire. For example, U.S. Pat. No. 7,116,548 to Satterfield, Jr., et al. and U.S. Patent Publication No. 2005/0237698 to Postage, et al. each describe various different configurations of multi-wire capacitors. Unfortunately, however, these solutions are still not fully satisfactory in achieving ultralow ESR levels. This is particularly evident in capacitors that employ high specific charge (“CV”) powders. Namely, such powders tend to shrink away and separate from an embedded anode wire during sintering, which can reduce the degree to which the wire is bonded to the particles of the anode body and increase ESR. This problem is actually compounded when multiple anode wires are employed because the total area of poor bonding increases.
As such, a need currently exists for an improved solid electrolytic capacitor that can exhibit a low ESR.